Also 100%+ weaken. 90% weaken is enough but adding that extra 30% weaken also helps. At 90%, weaken will trigger 90% of the time but at 100%+ weaken, it triggers 100% of the time.
Effective mythic makes sure your element attack is always effective aka enemy element weakness.
It seems like the negative resist theory is true after testing to find out.
I found that at -120%+ weaken, my damage exactly doubled. Well actually by ×2.2 on enemies with no resist or weakness to an element or immune. Exactly doubled is at 100% weaken.
This is because the nominal damage 1 becomes 1+1.2= 2.2x factor DMG. Why? Enemy from 0% resist becomes -120% resist which means they suffer 120% more DMG. I have tested throughout and thoroughly and the damage number results were correct. I noticed that when I put ignore resist afterwards, I noticed my damage going down to the normal DMG. I dealt 6 million DMG ignore resist and 12 million on 120% weaken as I've noticed. Well 6 million ×2.2 rather .
Of course if enemy resists your DMG without weaken, you lose your DMG by a certain %. If enemy resists your element, typically they reduce 25% of your full DMG. 25% damage Reduction to said element per say.
If enemy was already weak to an element, they already suffer -25% resist or take 25% more DMG . This is 1.25x DMG factor. Enemy has -25% resist on effective without weaken because 0%-25%= -25% resistance or takes 25% more DMG.
However with 120% weaken + Effective , -25%-120%= -145% resist or the enemy takes 145% more DMG or ×2.45 DMG. This means 6 million ×2.45 damage as I've noticed every time.
If enemy had resisted an element , typically they reduce your damage by 25% or your nominal damage is 0.75. However if you have 120% weaken , that is 25%-120%= -95% resit or 95% more DMG! Your damage becomes ×1.95 nominal instead of ×1.
With greatly resist, they reduce the elements DMG by 75% so that you only deal 25% of the original elements DMG or factor nominal number 0.25. That's basically 75% DMG Reduction .
However with 120% weaken, that's 75%-120%= -45% resist or they suffer 45% more DMG. Aka ×1.45 nominal DMG. This is worst case scenario here.
Immune enemies stop becoming immune to said elements DMG so that means with 120% weaken, 100%-120%= -20% resist or ×1.20 more DMG . Acts similarly to effective in DMG.
Obviously I can add more weaken , especially as safeguard for enemy maps that say increased enemy resist up to 50%. Theres also ascendant affix to add a guaranteed 62.5% reduce resists to enemies which is a bit like 62.5% weaken but it only works if enemy is arcane debuffed by arcane attack. Hirling + wrath or Iceburn is one way to utilise this though.
Then for rogue, there's noxious talent to help with weaken and smoke bombs .
On dots, the weaken buffing DMG by double is much more in the dot form and every frozen explosion of plagued poison cloud. The weaken at 100%+ would double any and every damage basically. At 200% weaken, you deal almost triple your normal DMG.
The maximum you could theoretically get is 200% weaken, 250% with Epiphany. Ascendant adds 62.5% reduce resists. Noxious adds 20% reduce resist for rogue or so. Effective makes sure you gain 25% reduce resist on enemy.
If 250% weaken + ascendant, you'd end up reducing enemy resist by 312.5% . This translates to 0-312.5%= -312.5% resist or ×4.125 more DMG. This would cost many slots to achieve however but that's the maximum theoretically. 200% weaken means enemies have -200% resist or ×3 your DMG which means enemies suffer 200% more DMG.
I also learnt of the Midlumer Ignore resist Vs Weaken guide and my own testing.
Yep: after having a look at Midlumer guide over and over again, his calculation matches to mine in some aspect but instead of showing -% resist, he does the other methods of math but it's still correct.